News Release Number: STScI-2006-32

Hubble Observations Confirm that Planets Form from Disks Around Stars

The full news release story:

More than 200 years ago, the philosopher Immanuel Kant first proposed that planets are
born from disks of dust and gas that swirl around their home stars. Though astronomers have detected more than 200 extrasolar planets and
have seen many debris disks around young stars, they have yet to observe a
planet and a debris disk aligned in the same plane around the same star. Now, NASA's Hubble Space Telescope, in collaboration with ground-based observatories,
has at last confirmed what Kant and scientists have long predicted: that planets form from
debris disks around stars.

The Hubble observations by a team of astronomers led by G. Fritz Benedict and
Barbara E. McArthur of the University of Texas at Austin show for the first time that a
planet is aligned with its star's circumstellar disk of dust and gas. The planet,
detected in 2000, orbits the nearby Sun-like star Epsilon Eridani, located 10.5
light-years from Earth in the constellation Eridanus. The planet's orbit is inclined 30
degrees to Earth, the same angle at which the star's disk is tilted. The results will
appear in the November issue of the Astronomical Journal.

The planets in our solar system share a common alignment, evidence that they were
created at the same time in the Sun's disk. But the Sun is a middle-aged star – 4.5
billion years old – and its debris disk dissipated long ago. Epsilon Eridani, however,
still retains its disk because it is young, only 800 million years old.

The Hubble observations also helped Benedict's team determine the planet's true mass,
which they calculate as 1.5 times Jupiter's mass. Previous estimates measured only the
lower limit, at 0.7 the mass of Jupiter. The planet, called Epsilon Eridani b, is the
nearest extrasolar planet to Earth. It orbits its star every 6.9 years.

"Because of Hubble, we know for sure that it is a planet and not a failed star," McArthur
explained. "Some astronomers have argued that a few of the known extrasolar planets
could be brown dwarfs because their precise masses are not known. If an object is less
than 10 Jupiter masses, it is a planet, not a brown dwarf.

McArthur was part of an earlier team at the University of Texas at Austin's McDonald
Observatory who discovered Epsilon Eridani b. They detected the planet using the radial-
velocity method, which measures a star's subtle motion toward and away from Earth to find
unseen companions.

Epsilon Eridani is a young and active star, so some astronomers claimed that what
appeared as a planet-induced wobble of the star could have been the actions of the star
itself. Turbulence in the atmosphere may have produced apparent velocity changes that
were intrinsic to the star and not due to a planet's influence.

The current Benedict-McArthur team calculated the planet's mass and its orbit by making
extremely precise measurements of the star's location as it wobbled on the sky, a technique
called astrometry. The slight wobbles are caused by the gravitational tug of the unseen
planet, like a small dog pulling its master on a leash. The team studied over a thousand
astrometric observations from Hubble collected over three years. The astronomers
combined these data with other astrometric observations made at the University of
Pittsburgh's Allegheny Observatory. They then added those measurements to hundreds
of ground-based radial-velocity measurements made over the past 25 years at McDonald
Observatory at the University of Texas, Lick Observatory at the University of
California Observatories, the Canada-France-Hawaii Telescope in Hawaii, and the
European Southern Observatory in Chile. This combination allowed them to accurately
determine the planet's mass by deducing the tilt of its orbit.

If astronomers don't know how a planet's orbit is tilted with respect to Earth, they can
only estimate a minimum mass for the planet. The planet's mass could be significantly
larger if the orbit were tilted to a nearly face-on orientation to Earth. The star would
still move toward and away from Earth slightly, even though it had a massive companion.

"You can't see the wobble induced by the planet with the naked eye," Benedict said. "But
Hubble's fine guidance sensors are so precise that they can measure the wobble. We
basically watched three years of a nearly seven-year-long dance of the star and its
invisible partner, the planet, around their orbits. The fine guidance sensors measured a
tiny change in the star's position, equivalent to the width of a quarter 750 miles away."

Epsilon Eridani has long captivated the attention of science fiction writers, as well as
astronomers. In 1960, years before the first extrasolar planet was detected, astronomer
Frank Drake listened for radio transmissions from inhabitants of any possible planets
around Epsilon Eridani as part of Project Ozma's search for intelligent extraterrestrial life
In the fictional "Star Trek" universe, Epsilon Eridani is considered by some fans to be the
parent star for the planet Vulcan, Mr. Spock's home.

No Vulcan or any other alien could live on this gas giant planet. If moons circled the planet,
they would spend part of their orbit close enough to Epsilon Eridani to have surface
temperatures like that of Earth, and possibly liquid water. However, the planet's looping,
"roller-coaster" orbit also would carry the moons far away from the star, a distance equal to
Jupiter's 500-million-mile separation from the Sun, where oceans would freeze. If a moon
were massive enough, like Saturn's giant moon Titan, it could have a sufficiently dense
atmosphere that would retain heat. Such an atmosphere would suppress wide swings in
surface temperatures, like covering up with a heavy blanket on a cold night. This could make
such a moon potentially habitable for life as we know it, Benedict said.

Although Hubble and other telescopes cannot image the gas giant planet now, they may be
able to snap pictures of it in 2007, when its orbit is closest to Epsilon Eridani. The planet
may be bright enough in reflected sunlight to be imaged by Hubble, other space-based
cameras, and large ground-based telescopes.